Bearings are used between the rotating and stationary parts of various types of machinery. High speed rotating machinery, such as motors, turbines, pumps, and compressors employ anti-friction elements to separate rotating and stationary components. Many traditional antifriction devices, such as ball and roller bearings, impose limitations on the size and speed of the rotating machinery. These bearings must be actively cooled and require oil lubrication sub-systems that provide a thin film of oil between the moving parts of the bearing. Oil lubrication sub-systems impose a burden on the primary machine and add a level of unreliability and inefficiency. Without oil, the metal-to-metal contact would cause the machine to grind to a halt. Use of oil-free bearings removes the need for the oil system thus reducing weight, maintenance, and complexity of the engine.
Oil-free operation is attained through the use of foil bearings. Foil bearings are unique anti-friction devices that utilize the working fluid of a machine as a lubricant, typically air for turbines and motors, and liquids for pumps, also act as a coolant to remove excess energy due to frictional heating in the bearings. Conventional foil bearings have been used for a number of years in high speed rotating machinery, air cycle machines for aircraft cabin pressurization, and other small turbomachinery. Foil bearings present an attractive alternative to ball or roller bearings for lightweight machines because they offer numerous system level benefits such as overall simplicity, reduction in weight, reduced friction, enhanced reliability, and zero oil contamination.
However, a primary technical challenge in the application of foil bearing technology to high speed rotating machinery is the dual use of the system process fluid as the hydrodynamic lubricant. Traditional foil bearings utilize forced cooling of the bearing and shaft, which results in reduced efficiency and reliability. Forced cooling is necessary because the heat capacity of the working fluid is low in comparison to the frictional heating of the bearing. There are several deleterious side effects caused by forced cooling. Firstly, the performance of the rotating machine is compromised because the forced cooling gas is taken from the main product of the machine. Secondly, the high flow rates needed to cool a foil bearing often lead to substandard hydrodynamic conditions of the foil bearing. Finally, forced cooling often masks the true stress of an operating foil bearing, which can cause bearing failure to be chaotic and unpredictable.
Specific limitations of conventional foil bearings include low load capacity, low damping, substantial friction, load capacity drop-off at high speed, and unpredictable failures.